What Type of Worth Should SEM Offer?

An other thread at Search Engine Watch forums named What is an SEO worth [in cold hard cash] seemed to start off on the topic of how much is a SEO worth by the hour, how much is a Directory lisiting consultant worth and how much are PPC consltants worth?

The thread kind of lead its way into comparing the SEO job with other types of jobs. The question began to turn around and now it seems the question being asked is as follows.

As an SEM, what type of service offering should I offer? Should I charge hourly? Should I charge based on spend? Should I charge monthly? Or maybe, better yet, charge on the CPA (cost per acquisition) model. In fact, I have spoken with several well known SEO/SEMs and they all agree with Mikkel's statement:

This [CPA] model has turned out to be far the most profitable for me (as calcualted by prfots per working hour) and the one where clients stick around for the longest time (in fact, I never had one single client terminate such a deal - yet).